The invention is relative to an apparatus for the selection of a tooth color for the manufacture of dental prostheses, with several plate-like color specimens which exhibit at least one curved section.
The color of an individual tooth is not distributed evenly and uniformly. The color, brightness and transparency decrease from the cutting edge or the masticatory surface of a tooth to the dental neck. The enamel exhibits a whitish-bluish color and the dentin, yellowish to brownish color tones. The dental neck and the root element are likewise of a yellowish-brownish color but usually darker than the corresponding dentin. The canines are generally somewhat darker than the other teeth.
The color shades of teeth can be very different in different individuals. In addition, inclusions of color can frequently be found on individual teeth, wherein the inclusions are shaded darker or brighter than the immediate surroundings. Finally, teeth become darker and loose translucency as a person ages.
In the restoration of a tooth or of a set of teeth, the tooth color in particular, in addition to the functional shaping, form, size and surface of a tooth, must be correctly selected. A tooth restoration should not only fit harmoniously into the arc of the teeth but it should also be adapted in its course of colors to the individual conditions of the adjacent teeth and the entire set of teeth.
If a tooth restoration, e.g. a crown, is to be manufactured, the tooth color to be achieved is customarily determined by means of a comparison of an analogous, natural tooth with a specimen tooth or with a color platelet. The manufacturer of the specimen teeth collates the tooth colors offered by him into a catalog which is usually called a color ring. The specimen teeth or color platelets are documented in adjacent delimited areas with neck material, dentin material and enamel material. The combination of these three materials on each specimen tooth is determined by the manufacturer, as is the available selection of tooth colors. A dentist can therefore only select a specimen tooth whose color is the most similar to the natural tooth of the patient.
The combination of the color tones of the dental neck, the dentin and the enamel are not reproducible for the dentist and the dental technician. In addition, inclusions of color, transparency effects, individual shadings and color peculiarities can not be comprehended. The tooth color effects frequently deviate from the color combination of the specimen. A dental technician reconstructs a restoration in the laboratory not in accordance with the peculiarities of the patient but rather in accordance with the abstract color patterns in front of him. An aesthetically satisfactory restoration is in most instances left to the experience of the dental technician, but for the most part to chance.
A great number of limiting conditions must be observed in the selection of the colors of a dental prosthesis.
First of all, the inherent color of the restoration, composed of the color tone, color intensity and color temperature is important. The spectral composition of the particular light source with which the restoration is illuminated plays a part. The natural tooth is not opaque but rather more or less translucent on its surface; for this reason, the transparencies and translucencies of the enamel and of the dentin should be taken into consideration. Light reflections result in bright spots and bright lines; therefore even the surface nature of a replacement tooth should agree with the surface of the adjacent teeth.
However, foreign colors of various origins also influence the selection of color. The color of the inner (pulp) and of the outer (gingiva, tongue, lip, cheek) soft parts in the vicinity of the tooth and the color of the adjacent hard tooth substances or restorations exert an influence. The surrounding buccal environment must also be taken into consideration; a drying out of the natural teeth has the result that they become brighter.
It is not possible with the previously known selection devices for tooth color to achieve a satisfactory result in a restoration, not even to mention an optimally functional and aesthetic dental prosthesis.
A metal carrier functions as support for the color-specimen tooth mass in polychromatic color rings, color plates or specimen teeth like those known e.g. from DE 34 29 927 A1 (HENNING), DE 35 03 084 A1 (HERRMANN) or DE 36 18 153 A1 (METHE). This metal influences the color of the specimen tooth by means of a foreign color shining from behind; such specimen systems are therefore not suitable for a restoration in full ceramics. The burning on of color specimens onto metal substrates also causes a loss of transparency and translucency and brings about an opacity; a restored tooth constructed according to such a specimen is therefore not lifelike.
In the case of polychromatic color rings, several colors are deposited onto a single specimen, to wit, the colors of the compartments of enamel, dentin and dental neck. The colors of the compartments can therefore not be indicated individually but rather only in the combination determined by the manufacturer.
It is not possible to comprehend the various individual colors or colors of an individual tooth. It is not at all possible to seek out the color shade of individual inclusions of color in the tooth. Finally, there is no way in the case of the known color rings to supply the dental technician with data about the transparency and the translucency of a tooth.
In the known specimens, the base color is the base color of the dentin. The colors of the enamel and of the dental neck are layered thereover in an undefined thickness; and therefore, their color can not be reproduced in an unambiguous manner at a later date.
In general, the layer thickness of the color specimens is selected too high and addition, inhomogeneous. This results in erroneous evaluations when seeking out a color specimen because the color intensity and color shade are also dependent on the layer thickness. Thus, a shift in color can take place because individual wavelengths are reabsorbed or reflected in the specimen itself. The result can be a gray effect. This is brought about by means of a layering of a series of colors which result in a subtractive color mixture.
Most of the known color specimens are built up in a manner similar to a tooth. However, such a specimen can not be placed areally (i.e. contiguous with an exposed surface) on a tooth but rather there is a relatively large space between the specimen and the object, the tooth. The selection result is falsified by colors present in the vicinity because uncontrolled shining phenomena of light and color occur. The seeking out of a proper color is also made more difficult by the optical isolation between the specimen and the tooth because air has a refractive index of approximately n=1 and on the other hand a tooth and color specimen exhibit a refractive index of n=1.5 to 1.8.
Not only the size and the form but also the inflexible fastening of the specimen to a probe oppose an orderly determination of color. For this reason too a specimen can not be placed areally on a tooth. In the case of a few polychromatic specimen systems, even the color of a single compartment is greater than a natural tooth. The bulkiness of the specimens often makes their handling difficult for the user too.
As a result of the specified disadvantages, an "exact" seeking of the color requires far too much time. However, the eye adapts already after a quite short time and adjusts to the light conditions and the colors in the vicinity of the tooth. After the eye has adapted to a wavelength range, the "separating sharpness" of the perception of color in this wavelength range decreases sharply. The consequence thereof is the fact that the color is improperly selected.
Many color rings also suffer from the defect that different masses are used in the color specimens than are used subsequently in the restoration.
Monochromatic color rings come either as intensive color wheels or as ball color systems. Monochromatic color rings are known e.g. from DE 35 17 344 A1 (KRASS), DE 37 13 491 A1 (BOON) and EU 234 945 A2 (FORSYTH).
In the case of intensive color wheels, intensive colors serve for a fine tuning of base colors which had been roughly determined previously. Intensive colors are selected thereby which are intermingled with the normal tooth color. The difficulties already described above occur here too.
There is no possibility of precisely determining the total color impression, it is not possible to indicate the translucency and the transparency and also, the color of individual compartments can not be determined in a reproducible manner.
It is difficult for the user to precisely indicate the intensive colors for an individual color mixture.
In the case of intensive color wheels, no original masses are available in the mixing ratio used later.
The color specimens are burned onto an opaque carrier of plastic or metal. This results again in an influencing of color by the foreign color of the carrier shining through.
In the case of ball color systems, an areal placing of the color specimen on a tooth is totally impossible on account of the awkward shaping. Both the great average distance of the ball from the tooth as well as the punctiform contact of the specimen render a direct comparison of the color specimen with the tooth difficult. The color specimen is not only isolated very well optically from the tooth but it is also chromatically separated by means of shining from many different sources. The "correct" color is therefore practically impossible to find.
There is also no possibility of determining the transparency effect in ball color systems.
The complicated handling of such systems requires no further comment. The color rings which operate with templates like those known e.g. from DE 26 41 740 C2 (WIENAND) or EU 01 47 232 B1 (HALL) are particularly disadvantageous. The user must look through a window of a colored template onto the tooth and is supposed to compare the tooth color with the color of the template. Even the handling involving a living object, namely, a human patient, is not simple. A correct determination of color is not possible on account of the great distance of the specimen from the tooth and on account of the shadow effect of the frame-shaped specimen.
Moreover, the eye of the tester tends to concentrate on the larger colored area of the template frame than on the small window and is thus involuntarily deflected and deceived chromatically.
DE 35 23 982 A1 (MUHLBAUER) involves the optical coupling of different layers of a color specimen for the adjustment of color in the repair of dental prostheses. To this end, an optical coupling liquid is introduced between the facing surfaces of adjacent layers. DE 35 23 982 A1 is relative to work performed in the dental laboratory but not to work performed in the mouth of a patient. This publication is concerned with the layered building up of color specimens, more precisely, with the building up of the layering (as regards the color) of artificial teeth. The use of optocoupling liquid between a color specimen element and the natural tooth surface in the mouth of a patient is not described by MUHLBAUER.
The initially mentioned apparatus for selecting a tooth color, is known from DE 37 31 254 A1 (ZFE). The color specimens known from this publication are on the order of magnitude of a complete tooth. The color specimens shown in this publication are curved in such a manner that an areal contact of the color specimens on the tooth, that is, on that (exposed) part of the tooth which is generally used for a color test, is not possible. As is already known from the state of the art, the color specimen taught in this publication also consists of a combination of colors, determined by the manufacturer, of differing tooth compartments (enamel, dentin and dental neck colors). For the rest, the carrier known from this publication for the color specimen consists of metal and only the outer, stained layer provided for the selection of color consists of facing mass.